A bearing unit rotatably supports a shaft and is typically arranged in a rotary drive such as a fan motor.
A bearing unit of the type under consideration is used with an I-shaped shaft (also referred to as straight shaft) that is rotatably supported by using lubricating oil. Japanese Patent Publication No. 3265906 describes such a bearing unit. The bearing unit 540 disclosed in the patent publication has a configuration as shown in FIG. 1 of the accompanying drawings. Referring to FIG. 1, the bearing unit 540 rotatably supports a shaft 541 by means of a radial bearing 542 and a thrust bearing 543. The radial bearing 542 is supported by a holding member 544 while the thrust bearing 542 is supported by a bottom plate 545. An anti-shaft-release member 546 is fitted to the shaft 541.
Since it is impossible to insert the anti-shaft-release member 546 in the last step of assembly due to the structure of the bearing unit 540, it is necessary to employ a step of mounting the thrust bearing 543 and the bottom plate 545 in the holding member 544 as the last assembling step. In other words, because of the use of such an assembling step, the housing of the bearing unit 540 needs to comprise a plurality of members, which include the holding member 544 and the bottom plate 545, for the purpose of providing the shaft 541 with the anti-shaft-release member 546. Then, it is difficult to completely seal the binding section 548 of the holding member 544 and the bottom plate 545. As a result, the lubricating oil filled in the inside can leak out.
Additionally, end facet 547 of the radial bearing 542 is exposed to the outside and lubricating oil can highly possibly disperse and evaporate from there.
Since the known bearing unit 540 comprises the anti-shaft-release member 546 for the shaft 541, the housing that takes the role of surrounding the bearing unit 540 and preventing lubricating oil from leaking and dispersing is formed by a plurality of members. Therefore, there is a problem that lubricating oil is apt to leak out through the binding sections of various members of the housing. There is also a problem that such a bearing unit needs to be prepared by way of complex steps to consequently raise the manufacturing cost.
Another known proposed bearing unit employs as a bearing means for supporting a shaft made of metal such as stainless steel a radial bearing and a thrust bearing that are sintered and oil-impregnated bearings or hydrodynamic fluid bearings and supported by respective housing members made of metal such as brass, of which the radial bearing is provided with a seal member for minimizing the leakage of the lubricating oil filled in an inner peripheral section thereof. With the proposed bearing unit, the shaft is rotatably supported by the radial bearing and the thrust bearing so that the shaft is supported and rotatable relative to the housing members.
With this bearing unit, lubricating oil is indispensable for the shaft to revolve smoothly and prevented from leaking out by a seal member. Lubricating oil can ooze out through every gap and leak to the outside of the bearing unit to shorten the service life of the bearing unit. Therefore, the binding sections of members of the bearing unit need to be hermetically sealed. For this purpose, the applicant of the present invention proposed a bearing unit wherein the binding section of a metal-made housing member and a seal member is hermetically sealed by means of a UV-ray set type adhesive agent in Japanese Patent Application No. 2001-289568 and another bearing unit wherein a resin-made housing member is used and integrally molded with a seal member in Japanese Patent Application No. 2002-34331.
However, with such bearing units, it is difficult to satisfactorily maintain the stability and the reliability.
For example, when a housing member and a seal member are prepared separately, it is difficult to completely bond or bind them together and hence to reliably prevent lubricating oil from leaking through the binding section thereof. Additionally, it is a highly complex and difficult operation to uniformly apply a polymeric packing material such as adhesive to the entire periphery of the binding section. It is also difficult to see if the binding section is completely sealed without micro-gaps. The net result will be either unsatisfactory reliability or prohibitive cost.
Leakage of lubricating oil cause to result in an unstable service life, a reduced reliability of the bearing unit and/or adverse effects (such as chemical attack phenomenon) on the externally disposed components of the bearing unit. For instance, when such a bearing unit is applied to a hard disc drive (HDD), leakage of lubricating oil containing organic substances cause to give rise to stiction and haze (a clouded disc surface).
When a housing member and a seal member are integrally molded, no gap is produced between them. However, if the gap between the seal member, which is now part of the housing member, and the shaft needs to be minimized when the housing is molded with the shaft supported by a bearing unit, it is difficult to guarantee the accuracy of minimizing the gap. For instance, any variance in the gap between the seal section and the shaft can influence the level of the oil surface and hence, if the volume of oil is large, lubricating oil cause to disperse to the outside when the oil temperature rises and/or when the ambient pressure changes.
The PCT Application Laid-open No. WO03/027521 describes another known bearing unit adapted to rotatably support a rotary shaft. Referring to FIG. 2 of the accompanying drawings, the bearing unit 660 is able to reliably prevent viscous fluid from leaking to the outside of the housing that is filled with viscous fluid even when the internal pressure of the housing changes due to a change in the environmental factors including atmospheric pressure and/or ambient temperature.
The bearing unit 660 comprises a radial bearing 644 and a thrust bearing 650, by which a rotary shaft 641 is rotatably supported, air releasing passage sections 662 being arranged between a housing 661 that is filled with lubricating oil 653 and the inner peripheral surface of an outer sleeve 667 that covers the housing 661.
The air releasing passage sections 662 are provided in order to prevent lubricating oil 653 from leaking to the outside of the bearing unit 660 when the atmospheric pressure falls due to an altitude change and the air inside the housing 661 expands. The housing 661 may be provided with one or more than one air releasing passage sections 662. In the illustrated bearing unit 660, three air releasing passage sections 662 are arranged on the outer periphery of the housing 661 and angularly separated from each other at regular intervals. The air releasing passage sections 662 can be formed in a simple manner when the housing 661 is produced by outsert molding along with the thrust bearing 650 that contains the radial bearing 644. If the air releasing passage sections 662 have a relatively complex profile, they can be formed when the housing 661 and the thrust bearing 650 are molded by using synthetic resin.
With the provision of such air releasing passage sections 662, it is possible to release the air that comes into the bearing unit when the rotary shaft 641 is inserted into the radial bearing 644 and placed in position.
In the bearing unit 660, each of the air releasing passage sections 662 has a first passage way 663 and a second passage way 664. The first passage way 663 is formed to extend along a radial direction of the housing 661 from an internal space 665 that is located near the thrust bearing 650. The inner end of the first passage way 663 is connected to the internal space 665 where the thrust bearing 650, which projects from the bottom closure section 647 of the housing 661, is located. The outer end of the first passage way 663 is connected to the second passage way 664. The second passage way 664 is exposed to the outer peripheral surface of the housing 661 and extends along the axial direction of the housing 661. While the air releasing passage section 662 including the first passage way 663 and the second passage way 664 has a relatively complex profile, it can be formed with ease when the housing 661 and the thrust bearing 650 are molded from synthetic resin.
As the bearing unit 660 is provided with the air releasing passage section 662, the inside of the housing 661 is not hermetically sealed. Therefore, the internal static pressure of the housing 661 does not fall when the rotary shaft 641 revolves relative to the housing 661 and hence the air remaining in the housing 661 does not expand to force out the lubricating oil in the inside.
The bearing unit 660 communicates with the outside by way of a plurality of sections including the air releasing passage sections 662 and the exposed part of the rotary shaft 641. In other words, the former operate as air inlet port while the latter operates as air outlet port so that lubricating oil cause to disperse when impact is applied to the bearing unit 660. In short, the bearing unit 660 is vulnerable to impact.
Japanese Patent Application Laid-Open Publication No. 2000-352414 describes still another bearing unit for rotatably supporting a rotary shaft. As shown in FIG. 3 of the accompanying drawings, the bearing unit 680 of the above cited patent document is adapted to rotatably support a rotary shaft 681 and comprises a radial bearing 682 for supporting the rotary shaft 681 in a peripheral direction, a thrust bearing 683 for supporting the rotary shaft 681 in a thrusting direction and a housing 685 containing the radial bearing 682 and the thrust bearing 683.
The radial bearing 682 of the bearing unit 680 operates as hydrodynamic fluid bearing with the lubricating oil filled in the housing 685, which lubricating oil is viscous fluid. Dynamic pressure generating grooves 684 are formed on the inner peripheral surface of the radial bearing 682, along which the rotary shaft 681 is inserted.
The radial bearing 682 is provided on the outer peripheral surface thereof with groove-shaped axial air passage ways 686 and groove-shaped radial air passage ways 687 that are used as air releasing passage ways when inserting the shaft member into the inner space of the bearing main body.
As shown in FIG. 3, housing 685 that contains the radial bearing 682, which by turn supports the rotary shaft 681, has a bottomed hollow cylindrical profile so as to surround the lateral surface and the bottom surface of the radial bearing 682, which also has a hollow cylindrical profile.
A compressed resilient body 688 is arranged in the upper opening of the housing 685 so as to be pressed against the inner peripheral surface of the housing 685 to operate as caulking member.
The rotary shaft 681 is inserted into the housing 685 of the bearing unit 680 that is caulked by the resilient body 688. At this time, air is released from the inside of the housing 685 to the outside by way of the axial air passage ways 686 and the radial air passage ways 687 so that the operation of inserting the rotary shaft 681 can be conducted smoothly.
Since the bearing main body is surrounded by two members of a housing 685 and a resilient body 688 in the bearing unit 680 of FIG. 3, lubricating oil cause to ooze out through the junction of the housing 685 and the resilient body 688.